This invention relates to the field of fluid flow rate control and metering, and more particularly, relates to flow rate control and metering of shear sensitive liquids.
Polymer solutions are probably the most commonly used shear sensitive liquids. Some liquids such as polyacrylamide solutions which are used in oil well drilling and in supplemental oil recovery operations are highly sensitive to the effects of shear. Many other liquids are also subject to substantial deterioration of shear degradation, for example, CMC (carboxymethyl-cellulose) solutions used in oil well drilling and in the food industry, latex polymers used in the manufacture of paints, solutions of biopolymers such as guar gum and polysaccharides used as flooding agents in supplemental oil recovery, some lubricating oils, and some lotions and salves used in cosmetics.
Shear sensitive polymer solutions are frequently used in chemical processes and in various phases of oil production to provide a liquid having desired high viscosity properties. For instance, during certain types of secondary and tertiary recovery of oil, a polymer solution such as polyacrylamide, is injected through the input wells, as a flooding agent, to displace oil trapped in the adjacent geological formation. High viscosity polymer solutions tend to flow in a cohesive manner, that is, the volume of polymer solution tends to flow uniformly through the geological formation thereby avoiding separate and independent channeling of polymer solution through the geological formation. Accordingly, flooding with high viscosity polymer fluids is often used to enhance oil recovery by displacing a generally larger portion of trapped oil than would otherwise be displaced by a liquid having a relatively lower viscosity.
Currently, polymer solutions are pumped to input wells by one or more large positive displacement pumps. Supply pressure at each input well may be as high as 2500 psig, however, the required discharge pressure into the input well may be as low as 0 psig, depending upon formation pressure and the desired flow rate of the polymer solution. If the polymer solution is shear sensitive, it is necessary to provide adequate means for flow rate control with a high pressure drop and without causing viscosity degradation of the liquid. It is also desirable to provide adequate means for flow rate metering at or near each input well.
Typically, shear stressing is caused by a high pressure gradient which may be directly imposed upon a liquid by conventional flow controlling means or by conventional turbine metering means. Unfortunately, shear stressing damages shear-sensitive polymer solutions by a phenomenon known as physical (shear) degradation. Significant shear degradation will occur in such liquids with any substantial reduction in pressure through conventional flow control means. The pressure drop across a valve or flow control means, which will be considered herein to result in a substantial or significant pressure differential, will vary depending upon the composition of the shear-sensitive liquid, the flow rate through the valve, and other variables. Nevertheless, for most shear-sensitive liquids, a pressure differential of about 50 psi across a valve member is a substantial pressure drop resulting in significant degradation of the liquid. Thus a pressure differential of about 50 psi is considered to be a substantial pressure differential with respect to the shear sensitive liquids referred to herein.
Physical degradation of shear-sensitive polymer solutions used for secondary and tertiary oil recovery operations is very undesirable because it generally causes a dramatic decrease in polymer solution viscosity. As previously mentioned, a suitable secondary and tertiary oil recovery flooding agent should have high viscosity. However, loss of viscosity of polymer solutions caused by conventional flow rate controlling means and conventional turbine metering means is often so great that the effectiveness of the polymer solution as a flooding agent is diminished or even destroyed.
For example, a throttle valve shear test was conducted using a conventional orifice-type throttle valve and an 800 ppm concentration polyacrylamide polymer solution. The pressure of the solution of the high-pressure side of the throttle valve was held substantially at 2300 psig while the solution flow rate was varied by adjusting the throttle valve. Approximately 50% of the solution viscosity was permanently lost when the solution was subjected to a 600 psig pressure drop across the throttle valve.
Because of the shear degradation problem, conventional means for controlling high-pressure drop flow rate, such as conventional orifice-type throttle valves or flow rate control valves (adapted to maintain constant flow with variable pressure drop across the valve), and conventional means for metering flow rate, such as in-line turbine meters, are unsatisfactory for service of polymer solutions for various uses requiring high viscosity liquids. They are clearly unsatisfactory at the well head during secondary and tertiary oil recovery operations because they can cause substantial shear stressing of the polymer solutions.
It has been suggested that a polymer solution be used that has a polymer concentration high enough to compensate for the polymer viscosity loss caused by conventional flow rate control valves. However, in a 600 psig pressure drop similar to the aforementioned example, roughly 30% more polymer would be required to maintain a 60 cp solution, rather than a 30 cp solution. Currently, such an approach is quite costly and, therefore, undesirable.
Accordingly, there is need of an apparatus and a method for substantially reducing the pressure of a liquid and for controlling and metering the flow rate thereof without causing significant shear stresses on the liquid. For various reasons, however, prior art attempts to provide such an apparatus have proved to be inadequate and/or very expensive.
Some of the prior art flow rate control devices which have been developed to avoid damaging polymer solutions are cumbersome, requiring piping disassembly or component replacement to adjust flow rate.
An example of such a device utilizes a plurality of copper tubing coils. Copper tubing of various lengths and internal diameters are fashioned as coils. The coils are manifolded in series with the flowline between the pump and the input wellhead so that polymer solution must pass through the coils. A low gradient pressure drop is imposed upon the polymer solution by passing it through the coils, thereby avoiding high shear stresses which would otherwise damage the solution.
This copper tubing device is expensive. Moreover, the pressure differential across and the flow rate through the coils of this device may be controlled solely by varying the combination of length and internal diameter of copper tubing used, which is usually accomplished by changing the coils. Proper coil sizing is dependent upon other variables such as solution pressure upstream and downstream from the coils and the viscosity of the solution. Of course, changing the coils is inconvenient and time consuming and interrupts the flow of solution to the input wellhead so that substantial slowing of the recovery operation due to repeated coil replacement is likely if there is significant fluctuation of the aforementioned variables.
Another polymer solution rate control device comprises a particular length of flowline section packed with spherical glass beads which provide a low gradient pressure drop as the solution flows past the glass beads. However, in addition to the problem of repeated replacement caused by the device's flow rate dependency upon upstream and downstream pressure and solution viscosity, sizing of the device must be determined experimentally, thus further slowing the recovery operation.
A more complete description of these devices may be found in:
"Operational Problems in North Burbank Unit Surfactant/Polymer Project", Journal of Petroleum Technology, (January, 1980), pp. 11-17; "Micellar-Polymer Injection System Has Special Features", The Oil and Gas Journal, (Oct. 3, 1977), pp. 79-85; and SPE/DOE 9826, "Polymer Augmented Water-flooding at the West Yellow Creek Field: Recovery and Cost Experience",
which are hereby incorporated by reference.
Polymer solution rate control devices of the prior art cannot detect and record flow rates of polymer solutions which they service. Therefore, present secondary and tertiary oil recovery operations require the additional expense and complication of a suitable metering means that will not significantly damage shear sensitive polymer solutions.
The disadvantages of prior art are overcome by the present invention which provides an improved apparatus and method for controlling and metering the flow rate of a liquid with a substantial upstream and downstream pressure differential across the device without causing significant shear stresses to act on the controlled liquid which is conveyed therethrough.